Radiation detection device and radiation detection method for nuclear medical diagnosis apparatus

ABSTRACT

In case of performing so-called “PET acquisition”, a nuclear medical diagnosis apparatus according to the present invention does not set only gamma rays which imparted predetermined energy values (for example, energy values of and above a Compton edge), as subjects for coincidental counting acquisition (coincidence acquisition), but it discriminates also further gamma rays which caused a photoelectric effect after having undergone Compton scattering once in a radiation detection unit, so as to set them as subjects for the coincidental counting acquisition. Concretely, if the added value of energy values observed in two detection elements is “near 511 keV”, the existence of the further gamma ray is presumed, and the acquisition thereof is possible. Besides, that one of the two detection elements as to which energy near the Compton edge is observed can be specified as the incident position of the gamma ray on the radiation detection unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation Application of PCT Application No.PCT/JP01/08656, filed Oct. 1, 2001, which was not published under PCTArticle 21(2) in English.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-304705, filed Oct.4, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a radiation detection device anda radiation detection method in a nuclear medical diagnosis apparatus.

[0005] 2. Description of the Related Art

[0006] Heretofore, there has been provided a nuclear medical diagnosisapparatus wherein a subject is dosed with a drug which is labeled with aradioisotope (hereinbelow, sometimes abbreviated to “RI”), and the stateof an RI distribution in the body of the subject is imaged on the basisof a result which is obtained by sensing and measuring gamma raysemitted from the RI, by means of a radiation detection unit. Inparticular, an SPECT (Single Photon Emission Computed Tomography)apparatus has been extensively known as an apparatus or means forradiographing the above image as a three-dimensional distribution image(tomogram). Owing to such an image, the user of the apparatus or anoperator can confirm the situation of the interior of the body of thesubject for example, a morbid part, a blood stream or a fatty-acidmetabolic rate) without resorting to surgical means.

[0007] Besides, there has been known a PET (Positron EmissionTomography) apparatus wherein a plurality of such radiation detectionunits are included, and a positron emitting nuclide is utilized as theradioisotope, so as to image a pair of gamma rays which are emitted indirections of 180 degrees at the annihilation of a positron by combiningwith an electron, and which are coincidentally detected by the pluralityof radiation detection units (as a coincidental counting measurement, orthe coincidence acquisition of the gamma rays). Incidentally, there hasalso been known a so-called “combined SPECT/PET apparatus” which iscapable of implementing both PET and SPECT by an identical system.

[0008] By the way, in the above apparatus, a suitable energy range (orenergy window) is usually set, whereby only gamma rays having energylevels within the range are acquired. Here, the suitable energy range isset as, for example, “energy of and above a Compton edge”, and all thegamma rays corresponding thereto are acquired to form the basis of theimaging. Thus, gamma rays having given rise to a photoelectric effectcan be utilized as principal basic data on the occasion of the imaging.

[0009] Herein, the radiation detection unit fundamentally has thefunction of receiving the incidence of the gamma rays, and convertingthe gamma rays into electric signals (easy of handling) while mirroringthe incident positions and energy levels thereof. The practicableaspects of the radiation detection unit are of two broad sorts called a“scintillation camera” and a “semiconductor detector”.

[0010] The scintillation camera is chiefly constituted by a scintillator(made of, for example, an NaI crystal, BGO, or LSO) and a photomultiplier tube (PMT). According to this, the gamma rays incident on thescintillator are converted into light signals, which are converted intothe electric signals by the photo multiplier tube. On the other hand,the semiconductor detector is so constructed that a plurality ofsemiconductor detection elements (of, for example, CdTe or CdZnTe) inwhich the incidence of the gamma rays contributes to the creation ofcharges (that is, the generation of the electric signals) are arrayed,for example, planarly (in the shape of a matrix) and discretely.

[0011] Meanwhile, in the nuclear medical diagnosis apparatus asexplained above, a theme to be stated below is generally existent. It isto enhance the efficiency of the gamma-ray acquisition in the radiationdetection unit. The reason therefor is that, since the result of thegamma-ray acquisition forms the basis of the imaging as stated above,usually a higher acquisition efficiency is more favorable in order toguarantee the image quality and preciseness of the imaging.

[0012] In this regard, in a case where the scintillation camera isutilized as the radiation detection unit and where its scintillator ismade of the BGO, LSO or the like, stopping power for the gamma rays iscomparatively high. Therefore, most of the incident gamma rays give riseto the photoelectric effect within the scintillator, and the gamma rayscan be acquired without being wasted (that is, all the energy can beimparted into the scintillator), so that the problem of the acquisitionefficiency can be said less serious.

[0013] However, in a case where the semiconductor detector is used, thestopping power is comparatively lower than in the scintillator, and manyof the incident gamma rays undergo transmission etc. without causinginteractions with the detection elements. Besides, although it isunreasonable to go to the extent of saying that the gamma rays aretransmitted through the detection elements without imparting any energythereto, it can be said by way of example that the gamma rays aretransmitted after causing Compton scattering, or that they give rise tothe photoelectric effect after having caused the Compton scattering.That is, in the case of utilizing the semiconductor detector, it isusually difficult to impart the energy of all the gamma rays to thedetection elements, and the acquisition efficiency for the gamma raysaccordingly lowers. Therefore, the quality of the image of the gammarays has also been affected. Incidentally, when the thickness of thesemiconductor detection elements is enlarged in order to heighten thestopping power with the intention of eliminating such a drawback, theCompton scattering occurs a plurality of times, and it becomesimpossible to specify the incident positions of the gamma rays.

[0014] Besides, even with the scintillation camera, in a case where thescintillator is made of NaI or the like, the stopping power is usuallylow, and hence, there has been the possibility that the same problem asstated above will be similarly posed.

[0015] By the way, such a fact becomes more problematic in a case wherethe gamma rays are high in energy. Besides, in such a case, an attemptto heighten an acquisition efficiency for the high-energy gamma rays isrecognized as the general theme irrespective of the sorts of thescintillation camera and the semiconductor detector as stated at thebeginning of this section.

[0016] The present invention has been made in view of the abovecircumstances, and has for its object to provide, in a radiationdetection unit, a radiation detection device for a nuclear medicaldiagnosis apparatus as is capable of enhancing an acquisition efficiencyfor gamma rays of high energy.

BRIEF SUMMARY OF THE INVENTION

[0017] The present invention has adopted the following means in order toaccomplish the object:

[0018] A radiation detection device for a nuclear medical diagnosisapparatus as defined in claim 1 is a radiation detection device for anuclear medical diagnosis apparatus having radiation detection means fordetecting gamma rays emitted from a radioisotope in a body of a subject,by a plurality of radiation detection cells, characterized in that theradiation detection means is made radiation detection means fordiscriminating and acquiring gamma rays which imparted predeterminedenergy, and for discriminating and acquiring also gamma rays whichcaused a photoelectric effect after having undergone Compton scatteringonce within the radiation detection means.

[0019] Besides, a radiation detection device for a nuclear medicaldiagnosis apparatus as defined in claim 2 is a radiation detectiondevice for a nuclear medical diagnosis apparatus having at least tworadiation detection means for detecting gamma rays emitted from aradioisotope in a body of a subject, by a plurality of radiationdetection cells, characterized in that, in a case where the gamma raysare gamma rays emitted in directions of 180 degrees at combination of apositron and an electron, and where coincidental counting acquisition isperformed for the gamma rays coincidentally detected in two of theradiation detection means, the radiation detection means is maderadiation detection means for discriminating gamma rays which impartedpredetermined energy, so as to submit them to the coincidental countingacquisition, and for discriminating also gamma rays which caused aphotoelectric effect after having undergone Compton scattering oncewithin the radiation detection means, so as to submit them to thecoincidental counting acquisition.

[0020] Further, a radiation detection device for a nuclear medicaldiagnosis apparatus as defined in claim 3 is the device as defined inclaim 1 or 2, characterized in that the radiation detection means is asemiconductor detector.

[0021] In addition, radiation detection devices for a nuclear medicaldiagnosis apparatus as defined in claims 4 through 7 are respectivelythe devices as defined in claim 2, characterized in that, in a casewhere the Compton scattering and the photoelectric effect arerespectively caused in adjacent ones of the radiation detection cells bythe gamma ray, the gamma ray is set as a subject for the coincidentalcounting acquisition (claim 4); characterized in that energy quantitiesimparted to the radiation detection cells through the Compton scatteringand the photoelectric effect are confirmed, thereby to specify anincident position of the gamma ray on the radiation detection unit(claim 5); characterized in that the radiation detection cells aredivided into a plurality of groups, and that the discrimination isperformed every group (claim 6); and characterized in that, in thecoincidental counting acquisition of the gamma ray which underwent theCompton scattering once within the radiation detection means, theradiation detection cell as to which energy of the gamma ray fallswithin a range of 166 keV-300 keV is regarded as an incident position ofthe gamma ray (claim 7).

[0022] Besides, a radiation detection device for a nuclear medicaldiagnosis apparatus as defined in claim 8 is a radiation detectiondevice for a nuclear medical diagnosis apparatus having radiationdetection means for detecting gamma rays emitted from a radioisotope ina body of a subject, by a plurality of radiation detection cells,characterized by comprising a coincidentality decision portion whichselects two gamma rays coincidentally detected by two of the radiationdetection cells, and an added-energy discrimination portion whichdiscriminates the two gamma rays in a case where an added value ofenergy values concerning the two gamma rays is equal to a predeterminedvalue.

[0023] Further, claims 9-16 feature radiation detection methodsrespectively characterized by steps which are achieved by theconstructions corresponding to the preceding claims 1-8 in succession.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0024]FIG. 1 is an outline diagram showing an example of construction ofa radiation detection device for a nuclear medical diagnosis apparatus(hereinbelow, simply termed “nuclear medical diagnosis apparatus”)according to an embodiment of the present invention.

[0025]FIG. 2 is an outline diagram showing an example of construction ofa gamma-ray energy discrimination unit shown in FIG. 1.

[0026]FIG. 3 is a flow chart showing the flow of the processing ofgamma-ray acquisition in this embodiment.

[0027]FIG. 4 is an explanatory diagram for explaining the phenomenon ofCompton scattering.

[0028]FIG. 5 is a graph showing the energy spectra of a pair of gammarays, wherein the axis of abscissas represents the value of energy,while the axis of ordinates represents the number of counts (a countvalue).

[0029]FIG. 6 is an explanatory diagram for explaining a case where agamma ray gives rise to a photoelectric effect after having undergoneCompton scattering once, in a radiation detection unit.

[0030]FIG. 7 is an outline diagram showing an example of construction ofa gamma-ray energy discrimination unit which is different from the unitshown in FIG. 2.

[0031]FIG. 8 is an outline diagram showing an example of construction inwhich detection elements in each radiation detection unit are dividedinto a plurality of groups, and which has an energy discriminationportion every group.

[0032]FIG. 9 is an outline diagram showing an example of construction inthe case where three radiation detection units are disposed.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Now, the first embodiment of the present invention will bedescribed with reference to the drawings. FIG. 1 is an outline diagramshowing an example of construction of a nuclear medical diagnosisapparatus according to this embodiment. Referring to FIG. 1, the nuclearmedical diagnosis apparatus is constructed of radiation detection units(radiation detection means) 1, gamma-ray energy discrimination units 2,a coincidence decision unit 3, a data acquisition unit 4, an imagecreation unit 5 and an image memory 6.

[0034] Each radiation detection unit 1 has a general external view in aflat shape, and it is configured of a plurality of semiconductordetection elements 12 (termed “radiation detection cells” in the presentinvention, and hereinbelow called “detection elements”) which arearrayed discretely and in the shape of a matrix within an XY-planeindicated in FIG. 1.

[0035] Besides, as shown in FIG. 1, the radiation detection units 1 inthis embodiment are disposed in the number of two (refer to symbols 1Aand 1B in the figure, both units having the same constructions) so thatthe respective units may oppose to each other around a subject P who islying on the tabletop PL of a diagnostic table. PET acquisition to bestated later is permitted by utilizing the two radiation detection units1A and 1B. Further, the radiation detection units 1A and 1B are entirelysupported by a turning arm not shown, and they can rotate round thesubject P as indicated by an arrow A in the figure.

[0036] The detection elements 12 receive gamma rays emitted from aradioisotope with which the subject P is dosed, and they directlyconvert the gamma rays into electric signals. Each of the electricsignals contains position information and energy informationrespectively representing which position in the radiation detection unit1A or 1B the originating gamma ray was sensed at (in other words,“which” detection element 12 it was sensed at), and how much energy thecorresponding gamma ray has.

[0037] More concretely, a compound semiconductor; cadmium telluride(CdTe), for example, can be used for the detection element 12. Besides,the size of the detection element 12 can be set at, for example, 1.6mm×1.6 mm per area on which the gamma ray is incident. By the way, inthe present invention, a compound semiconductor CdZnTe may well beemployed for the detection element 12. Besides, in order to derive theelectric signal, the detection element 12 is additionally provided withan electrode not shown. The electrode has, for example, a Schottkystructure which is formed of platinum and indium, and the platinum sidethereof is used as an electrode for applying a high voltage, while theindium side thereof is used as an electrode for deriving the signal.Further, an insulating sheet or the like, not shown, is provided amongthe plurality of detection elements 12 disposed. In addition, thedetection elements 12 are respectively furnished with charge amplifiersand waveshapers (neither of which is shown), through which the aboveelectric signals are passed and outputted.

[0038] According to such a semiconductor detector, it is permitted todirectly the electric signals from the gamma rays, unlike in the use ofa scintillator. Moreover, the formation of the detection elements 12 isespecially adapted to be easily made smaller in size (that is, theelements can be packaged more highly), so that the essential resolution,namely, so-called “intrinsic resolution” of the elements can beheightened.

[0039] Meanwhile, referring to FIG. 1, the gamma ray energydiscrimination units 2 are disposed in two sets (A and B) incorrespondence with the fact that the two radiation detection units 1are disposed as stated above. Besides, as shown in FIG. 2, thepracticable construction of each of the gamma-ray energy discriminationunits 2A and 2B is constituted by a time stamp portion 21, a coordinatedetection portion 22, an energy discrimination portion 23, and acoincidentality decision portion 24 as well as an added energydiscrimination portion 25. Two signals (simple energy discrimination andadded energy discrimination) issued from each of the gamma-ray energydiscrimination units 2A and 2B shown in FIG. 1 are respectively obtainedby passing through the energy discrimination portion 23 and through thecoincidentality decision portion 24 as well as the added-energydiscrimination portion 25.

[0040] Referring to FIG. 2, the time stamp portion 21 stamps “when” theelectric signal outputted from the radiation detection unit 1A or 1B wassensed on the corresponding radiation detection unit 1A or 1B (=asensing time). Besides, the coordinate detection portion 22 specifies“which” detection element 12 of the radiation detection unit 1A or 1Bthe originating gamma ray of the electric signal was incident on, thatis, it extracts position information.

[0041] The energy discrimination portion 23 decides whether or notenergy information contained in the electric signal falls within apredetermined energy range. When the energy information is judged tofall within the range, the portion 23 issues an energy signalproportional to the energy of the gamma ray (a signal based on simpleenergy discrimination, as to which refer to FIG. 1), and when not, itdoes not issue any signal. Incidentally, the “energy range” stated herecan be set by the user of the apparatus in such a way, for example, thata minimum value and a maximum value, or a minimum value (or a maximumvalue) and a predetermined width having the minimum value (or themaximum value) as one end are designated as to energy values. Besides, aplurality of such energy ranges can be set, and concretely, “energy ofand above a Compton edge” or the like can be set as will be statedlater.

[0042] Besides, in a case where the electric signals outputted from anytwo detection elements 12 in either of the radiation detection units 1Aand 1B are recognized by the coincidentality decision portion 24 ashaving been “coincidentally” outputted (=in a case where gamma rays arerecognized as having been coincidentally detected in the two detectionelements 12), the added-energy discrimination portion 25 selects the twoelectric signals and adds up energy information items relevant to thetwo electric signals. When the sum of the addition is equal to apredetermined energy value (predetermined value), the portion 25 issuesan energy signal proportional to the sum energy of the two gamma rays (asignal based on added energy discrimination, as to which refer to FIG.1), and when not, it does not issue any signal.

[0043] Incidentally, the operation of the added-energy discriminationportion 25 will be stated in detail later. Besides, data passed throughneither of the energy discrimination portion 23 and the added-energydiscrimination portion 25 is discarded as shown in FIG. 2.

[0044] Referring back to FIG. 1, the coincidence decision unit 3 selectsfrom the individual signals passed through the gamma-ray energydiscrimination units 2A and 2B, ones whose sensing times stamped by thetime stamp portion 21 are “coincidental” in the respective radiationdetection units 1A and 1B, and it sends the selected data to the dataacquisition unit 4. Incidentally, “coincidental” stated here correspondsto, for example, a case where the differences of the sensing times arewithin “10 ns” or so. The data acquisition unit 4 accumulates the sentdata as gamma-ray data at any time. That is, in this embodiment, bypassing the signals through the time stamp portion 21 and thecoincidence decision unit 3, so-called “coincidence acquisition”(coincidental counting acquisition) is performed concerning the gammarays sensed by the two sets of radiation detection units 1A and 1B.Incidentally, the signals which are not recognized as being coincidentalare discarded as shown in FIG. 1.

[0045] The image creation unit 5 creates a planar image concerning an RIdistribution within the subject P or reconstructs a tomogram on thebasis of the gamma-ray data accumulated in the data acquisition unit 4,and the image memory 6 stores the created planar image or reconstructedtomogram therein. Incidentally, although not shown, an image displayunit or the like is connected after the image memory 6 so as to displaythe image stored in the image memory 6.

[0046] The operation and advantages of the nuclear medical diagnosisapparatus of the above construction will be described below.Incidentally, the present invention features in the case of performingthe PET acquisition, that, not only an image is formed on the basis ofgamma rays having caused only a photoelectric effect, as usual, but alsothe data of gamma rays causing the photoelectric effect after havingundergone Compton scattering once, among the gamma rays incident on theradiation detection units 1A and 1B, are utilized for imaging.Therefore, this point will be chiefly explained below.

[0047] First, as indicated at a step S1 in FIG. 3, the acquisition ofgamma rays emitted from a radioisotope RI being a positron nuclide (forexample, ¹¹C or ¹³N), with which the subject P has been dosed, isstarted (=the user of the apparatus gives an acquisition start command)to enter a gamma-ray acquisition mode. The gamma rays are pairs of gammarays which are emitted in the exact opposite directions (in directionsof 180 degrees) to each other when a positron and an electron combine toannihilate (refer to broken lines which stretch from the radioisotope RIwithin the subject P in FIG. 1). Owing to the gamma-ray emission, threesorts of interactions; the photoelectric effect, the Compton effect andelectron pair generation occur between the interiors of the radiationdetection units 1 or the detection elements 12 thereof and thecorresponding gamma rays.

[0048] Incidentally, as a premise for starting the gamma-ray acquisitionstated now, the user of the apparatus can set the range of gamma-rayenergy levels to-be-acquired (or an energy window), a time period forthe acquisition, the number of counts for the acquisition, etc. Also,description will be developed below about a case where “energy of andabove a Compton edge” is set as the energy range, namely, a range forthe acquisition in the energy discrimination portion 23.

[0049] By the way, the “Compton edge” concretely has a significance asstated below. First, the “Compton effect” is the phenomenon that, asshown in FIG. 4, an electron E in the detection element 12 and anincident gamma ray G come into elastic collision, whereby the electron Eis emitted, while the gamma ray G is scattered (at Gs in the figure). Onthis occasion, energy Ee is imparted to the electron E by the gamma rayG. That is, the electric signal to be detected is proportional to theenergy Ee. In this case, the momentum and energy of a system shown inthe figure are conserved as is well known, the energy can be expressedby introducing a scattering angle θ, as follows:

Ee=E/(1+m _(e) c ² /E(1−cos θ))  (1)

[0050] Here, m_(e) denotes the rest mass of the electron E, and cdenotes the velocity of light.

[0051] In this regard, in a case where the scattering angle θ is 180degrees in the above equation (1), that is, where the gamma ray Gscatters backward as indicated by symbol Gb in FIG. 4, cos θ=−1 holds,and the energy Ee exhibits a maximum value Ee max. Besides, the maximumenergy Ee max is generally called the “Compton edge”, and it becomes asfollows from Eq. (1):

E _(e) max=E/(1+m _(e) c ²/2E)  (2)

[0052] Owing to the above, that is, according to the definition of theenergy range as the “energy of and above the Compton edge”, it ispermitted to neglect the energy which has been imparted to the detectionelement 12 by the Compton effect, namely, to acquire an energy spectrumwhose principal component is a photoelectric peak involved in theoccurrence of the photoelectric effect. Incidentally, this is none otherthan “to acquire gamma rays having imparted predetermined energy toradiation detection means” stated in the present invention.

[0053] Now, referring back to FIG. 3, when data acquisition concerninggamma rays is started, the gamma rays sensed by the detection elements12 of the radiation detection units 1A and 1B are converted intoelectric signals containing their incident position information andenergy information by the operations of the detection elements 12 asindicated at a step S2 in FIG. 3. Besides, the electric signals are sentto the time stamp portions 21 so as to stamp sensing times, as indicatedat a step S3 in FIG. 3, and they are thereafter sent to the coordinatedetection portions 22 so as to extract the position information items ofthe gamma rays, as indicated at a step S4 in FIG. 3.

[0054] Subsequently, at a step S5 in FIG. 3, whether or not the energylevels of the gamma rays fall within the energy range preset in theabove, namely, within the “energy of and above the Compton edge” isjudged in the energy discrimination portions 23. Here, on condition thatthe energy levels of the gamma rays are judged to fall within the energyrange, whether or not the sensing times stamped in the above are equalin the respective radiation detection portions 1A and 1B, namely,whether or not the gamma rays were “coincidentally” sensed, is decidedby the coincidence decision unit 3 (step S7 in FIG. 3). Further, whenthe gamma rays are judged to have been sensed “noncoincidentally”, thedata are discarded (step S81 in FIG. 3), and when they are judged tohave been sensed “coincidentally”, the data are sent to the dataacquisition unit 4 (step S82 in FIG. 3).

[0055] On the other hand, at a step S6 in FIG. 3, regarding the electricsignals coincidentally outputted from any two detection elements 12 ineither the radiation detection unit 1A or 1B, whether or not the addedenergy value of these electric signals is equal to a “predeterminedvalue” set beforehand is decided by the added-energy discriminationportion 25. Incidentally, the electric signals recognized as having beencoincidentally outputted are selected on the basis of the sensing timesstamped by the time stamp portions 21, in the coincidentality decisionportions 24 before the step S6 in FIG. 3.

[0056] Here, the above “predetermined value” is set on the basis of abackground as stated below, in this embodiment. First, in general,regarding the pair of gamma rays as in this embodiment, the energyspectrum thereof is obtained as one shown in FIG. 5. Depicted in thefigure are a photoelectric peak P1 seen near an energy value of 511 keVwhich is peculiar to the gamma rays, spectra S1-S4 which arerespectively observed by first fourth Compton scatterings, a spectrum Sin the case where the spectra S1-S4 are simultaneously observed, etc.Besides, a part which looks like, so to speak, a “wall” near an energyvalue of 340 keV is the Compton edge stated above (symbol CE in thefigure) (that is, Ee max≅340 keV). By the way, it is directly seen fromFIG. 5 that the gamma rays participating in the photoelectric peak P1are chiefly acquired in the above energy discrimination portions 23.

[0057] Meanwhile, each gamma ray incident on the detection element 12undergoes the interactions, such as photoelectric effect and Comptoneffect, with this detection element 12 as stated above. In thisembodiment, among the gamma rays, the gamma ray which causes thephotoelectric effect after having undergone the Compton scattering onceis especially noted. Regarding such a gamma ray, a case is consideredwhere, as shown in FIG. 6 by way of example, it undergoes the Comptonscattering in a certain detection element 121 on the radiation detectionunit 1A or 1B and thereafter causes the photoelectric effect in“another” detection element 122.

[0058] Besides, in such a case, energy Ee below the Compton edge CE isimparted to the detection element 121, and energy involved in thephotoelectric effect is imparted to the other detection element 122 by ascattered gamma ray Gs (refer to FIG. 6) which has been deprived of theenergy Ee. Accordingly, assuming now that the Compton scattering in thedetection element 121 as shown in FIG. 6 be backward scattering, anenergy value obtained by the subtraction of about 340 keV being theenergy value of the Compton edge CE, from about 511 keV being the energyvalue of the gamma ray G, that is, energy “near 171 keV” is observed inthe other detection element 122.

[0059] These facts result in the following: If the added value of theenergy information items observed at any two detection elements 12 ineither the radiation detection unit 1A or 1B is near 511 keV (andbesides, the energy information is near 171 keV in one of the detectionelements), the existence of the detection element 121 and the otherdetection element 122 as stated above is presumed. In other words, theexistence of the gamma ray which has caused the photoelectric effectafter having undergone the Compton scattering once is presumed. That is,the “predetermined value” stated above signifies “near 511 keV”, andeven the gamma ray participating in such a behavior is set as a subjectfor the acquisition in order to form the basis of imaging in thisembodiment. Incidentally, it is indicated in FIG. 5 that a peak P2 near171 keV is certainly observed.

[0060] Incidentally, as explained with reference to FIG. 4, the Comptonedge CE is the maximum energy (=Ee max) which is lost (=which isimparted to the electron E) in the case where the gamma ray G undergoesthe Compton scattering. Therefore, whatever Compton scatteringphenomenon has occurred, an energy value at which the peak P2 as shownin FIG. 5 will be observed cannot become below near 171 (=511—“near340”) keV. The word “near” in “near 171 keV” stated now can define anapproximately appropriate range on the basis of the above circumstances.Incidentally, regarding the word “near”, reference should be had also tolater description.

[0061] Besides, in such a case, the electric signals are“coincidentally” detected from the two detection elements 12 (thedetection elements 121 and 122 in FIG. 6), so that the incident positionof the pertinent gamma ray in the radiation detection unit 1A or 1B isspecified as stated below. Energy values (=imparted energy quantities)observed in the two detection elements 12 are confirmed, whereupon thatone of the detection elements 12 as to which the peak P2 was “notobserved”, namely, the detection element 12 as to which an energy valuenear the Compton edge CE was observed (in FIG. 6, the detection element121) may be specified as the incident position. Incidentally, for thispurpose, positional coordinate data concerning the two detectionelements 12 is transmitted from the coordinate detection portion 22 tothe added-energy discrimination portion 25 as shown in FIG. 3, and theincident position (coordinates) is specified in the discriminationportion 25 on the basis of the data and the above idea.

[0062] The arithmetic operation as explained above is executed at thestep S6 in FIG. 3, or in the added-energy discrimination portion 25 inFIG. 2. Besides, the data thus discriminated is delivered to thecoincidence decision unit 2, likewise to the data passed through theenergy discrimination portion 23, and whether or not the correspondinggamma rays were “coincidentally” detected (=coincidence) in each of theradiation detection units 1A and 1B (at step S7 in FIG. 3). The datatransmission to the data acquisition unit 4 (step S82 in FIG. 3) or thedata discard (step S81 in FIG. 3) is executed on the basis of thedecision.

[0063] After all, in this embodiment, even the gamma rays which havecaused the photoelectric effect after having undergone the Comptonscattering once in either or both of the radiation detection units 1Aand 1B can be set as subjects for the acquisition. More concretely, inthis embodiment, coincidence acquisitions conforming to the followingthree patterns are carried out:

[0064] The first pattern is that gamma rays caused a photoelectriceffect in the radiation detection unit 1A (or 1B) and caused aphotoelectric effect “also” in the radiation detection unit 1B (or 1A),and they were coincidentally detected. The second pattern is that gammarays caused a photoelectric effect in the radiation detection unit 1A(or 1B) after having undergone Compton scattering once and caused aphotoelectric effect “also” in the radiation detection unit 1B (or 1A)after having undergone Compton scattering once, and they werecoincidentally detected. The third pattern is that gamma rays caused aphotoelectric effect in the radiation detection unit 1A or 1B and causeda photoelectric effect in the radiation detection unit 1B or 1A afterhaving undergone Compton scattering once, and they were coincidentallydetected.

[0065] Incidentally, it has been described above that the predeterminedvalue is set “near 511 keV”, and that the discrimination in theadded-energy discrimination portion 25 is based on whether or not theadded value of the energy information outputs coincidentally deliveredfrom the two detection elements 12 agrees with the predetermined value.However, it is more actual and more practicable to endow thepredetermined “value” with a certain degree of width.

[0066] By way of example, it is possible to pass the signal through theadded-energy discrimination portion 25 in a case where energy “166 keVto 350 keV” is observed in one of the two detection elements 12, whileenergy “330 keV to 526 keV” is observed in the other. That is, in thiscase, the predetermined value stated above has a width of “496-876 keV”,and the signal is passed through the added-energy discrimination portion25 when the added value of the observed energy information items lieswithin the range of the width. Incidentally, it is needless to say that,in the example stated now, the latter in which the energy value of “330keV to 526 keV” is observed is the detection element which caused theCompton scattering, while the former in which the energy of “166 keV to350 keV” is observed is the detection element which caused thephotoelectric effect after the Compton scattering. By the way, suchcircumstances are also contained in the concept “near” stated above tosome extent.

[0067] Thereafter, the gamma-ray acquisition as stated above iscontinued till the completion thereof (step S9 in FIG. 3). Whether ornot the acquisition has been completed, is determined by referring tothe time period for the acquisition or the number of counts for theacquisition as set beforehand. Besides, the acquisition can be sometimesended midway in compliance with a direct command given by the user ofthe apparatus.

[0068] As thus far described, according to the nuclear medical diagnosisapparatus in this embodiment, only the gamma rays which caused merelythe photoelectric effect are not set as subjects for acquisition, butthe gamma rays which caused the photoelectric effect after havingundergone the Compton scattering once are also set as subjects foracquisition, so that an acquisition efficiency for the gamma rays ofhigh energy can be enhanced. Besides, this advantage can be remarkablyenjoyed especially in the radiation detection unit 1A or 1B whosestopping power is comparatively low. Owing to these facts, in thisembodiment, image creation can be performed on the basis of moreinformation items, so that an image of higher precision and higherquality can be obtained.

[0069] Incidentally, it has been described in this embodiment that, inthe case where the gamma rays which caused the photoelectric effectafter the Compton scattering are to be acquired, the added-energydiscrimination portion 25 executes the arithmetic operations concerningthe discrimination as to “any” detection elements 12. The presentinvention, however, can be performed in an aspect where the operationsare executed as to “adjacent” detection elements 12. For this purpose,an example of construction in which an adjacency decision portion 2N isadded as exemplified in FIG. 7 may be adopted instead of theconstruction of the gamma-ray energy discrimination unit 2A or 2B shownin FIG. 2. Besides, position information is necessary for decidingwhether or not the detection elements are “adjacent”. It is thereforeindicated that the adjacency decision portion 2N in FIG. 7 receives anoutput from the coordinate detection portion 22.

[0070] Incidentally, the word “adjacent” stated here is a term intendedto signify the relationship between a certain noticed one 12 ofdetection elements 12 which are arrayed, for example, in the shape of atwo-dimensional matrix and the eight detection elements 12 which existaround the noticed detection element 12 (of course, when the detectionelement 12 arrayed at the peripheral edge or corner of the matrix isnoticed, the word “adjacent” signifies the relationship of the noticeddetection element 12 and the five or three detection elements 12).

[0071] In this way, arithmetic operations tacitly premised in the aboveembodiment as sweep the whole area of the radiation detection unit 1A or1B are dispensed with, so that an arithmetic speed can be enhanced. Bythe way, in view of the fact that most of the gamma ray which cause thephotoelectric effect after having undergone the Compton scattering once,usually give rise to the pertinent phenomenon between the adjacentdetection elements 121 and 122 a shown in FIG. 6, such restriction ofthe detection elements for the arithmetic operations as stated abovedoes not pose any serious problem (for example, that an acquisitionefficiency lowers drastically) in actuality.

[0072] Besides, from the viewpoint of enhancing an arithmetic speed, itis favorable to adopt an aspect in which the detection elements 12 onthe radiation detection units 1A and 1B in the above embodiment aredivided into a plurality of groups G1, . . . , G9 as shown in FIG. 8 byway of example, and an energy discrimination unit 2G capable ofexecuting arithmetic operations in units of the groups G1, . . . , G9 isincluded, whereby the arithmetic operations for added-energydiscriminations are executed in parallel. By the way, in FIG. 8, eachblock in the energy discrimination unit 2G has the same construction asshown in FIG. 2 or FIG. 7. In this way, each of the blocks may renderthe coincidentality decision for the two detection elements 12 as shownin FIG. 6 by way of example, as to only the corresponding one of thegroups G1, . . . , G9, so that the arithmetic speed can be enhanced forthe same reason as stated above. Incidentally, since the number ofgroups need not always be set at “9” in the present invention, it may,of course, be set at will.

[0073] Further, in the above embodiment, the so-called “semiconductordetectors” utilizing the semiconductor detection elements have beenexplained as the radiation detection units 1A and 1B. Of course,however, the present invention is not restricted to such an aspect, butit is similarly applicable to, for example, a case of utilizing ascintillation camera whose scintillator is made of NaI, BGO, LSO or thelike. Even in such a case, the advantage of enhancing an acquisitionefficiency for gamma rays of high energy is similarly enjoyed.Incidentally, on this occasion, especially in a case where thescintillator is made of NaI, the stopping power thereof is said to becomparatively low, and hence, the application of the present inventionis more advantageous.

[0074] Still further, the nuclear medical diagnosis apparatus accordingto the present invention is not restricted to the above example ofconstruction as shown in FIG. 1 and FIG. 2, FIG. 7 or FIG. 8 or theabove processing flow shown in FIG. 3. By way of example, in theforegoing, the two radiation detection units 1 (1A and 1B) are disposed,and they are arranged so as to oppose to each other with the subject Pinterposed therebetween. In some cases, however, it is allowed to adoptan aspect in which three radiation detection units 100A, 100B and 100Care disposed as shown in FIG. 9, or in which three or more radiationdetection units 1 are disposed. In such a case, for example, in the caseof FIG. 9, coincidence decisions (acquisitions) may be done between theradiation detection units 100A and 100B, 100B and 100C, and 100C and100A.

[0075] Besides, the above aspect in FIG. 3 is such that the gamma-rayacquisition and the coincidence decision are done, so to speak, inparallel. It is also allowed, however, to adopt instead of the aspect, aconstruction or a processing flow in which, after gamma rays have beenentirely acquired once on the basis of a predetermined acquisition timeperiod or acquisition count number, all the data thereof arecollectively submitted to coincidence decisions.

[0076] As described above, according to the nuclear medical diagnosisapparatus of the present invention, gamma rays which cause aphotoelectric effect after having undergone Compton scattering once arealso set as subjects for acquisition, whereby an acquisition efficiencyfor the gamma rays of high energy can be enhanced in a radiationdetection unit. This advantage becomes remarkable especially in a casewhere a semiconductor detector of comparatively low stopping power isutilized as the radiation detection unit. Besides, as a result, an imageto be created becomes preciser, and its image quality can be enhanced.

What is claimed is:
 1. In a radiation detection device for a nuclearmedical diagnosis apparatus having radiation detection means fordetecting gamma rays emitted from a radioisotope in a body of a subject,by a plurality of radiation detection cells; a radiation detectiondevice for a nuclear medical diagnosis apparatus, wherein: saidradiation detection means is radiation detection means fordiscriminating and acquiring gamma rays which imparted predeterminedenergy, and for discriminating and acquiring also gamma rays whichcaused a photoelectric effect after having undergone Compton scatteringonce within said radiation detection means.
 2. In a radiation detectiondevice for a nuclear medical diagnosis apparatus having at least tworadiation detection means for detecting gamma rays emitted from aradioisotope in a body of a subject, by a plurality of radiationdetection cells; a radiation detection device for a nuclear medicaldiagnosis apparatus, wherein: when the gamma rays are gamma rays emittedin directions of 180 degrees at combination of a positron and anelectron, and coincidental counting acquisition is done for the gammarays coincidentally detected in two of said radiation detection means,said radiation detection means is radiation detection means fordiscriminating gamma rays which imparted predetermined energy, so as tosubmit them to the coincidental counting acquisition, and fordiscriminating also gamma rays which caused a photoelectric effect afterhaving undergone Compton scattering once with in said radiationdetection means, so as to submit them to the coincidental countingacquisition.
 3. A radiation detection device for a nuclear medicaldiagnosis apparatus as defined in claim 1 or 2, wherein said radiationdetection means is a semiconductor detector.
 4. A radiation detectiondevice for a nuclear is medical diagnosis apparatus as defined in claim2, wherein, in a case where the Compton scattering and the photoelectriceffect are respectively caused in adjacent ones of said radiationdetection cells by the gamma ray, said radiation detection means isradiation detection means for submitting said gamma ray to thecoincidental counting acquisition.
 5. A radiation detection device for anuclear medical diagnosis apparatus as defined in claim 2, wherein, inaccordance with energy quantities imparted to the radiation detectioncells through the Compton scattering and the photoelectric effect, anincident position of the gamma ray on said radiation detection means isspecified.
 6. A radiation detection device for a nuclear medicaldiagnosis apparatus as defined in claim 2, wherein said radiationdetection cells are divided into a plurality of groups, and that thediscrimination is done every group.
 7. A radiation detection device fora nuclear medical diagnosis apparatus as defined in claim 2, wherein, inthe coincidental counting measurement and acquisition of the gamma raywhich caused the photoelectric effect after having undergone the Comptonscattering once within said radiation detector, the radiation detectioncell as to which energy of said gamma ray falls within a range of 166keV through 300 keV is set as a subject for acquisition as an incidentposition of said gamma ray.
 8. In a radiation detection device for anuclear medical diagnosis apparatus having radiation detection means fordetecting gamma rays emitted from a radioisotope in a body of a subject,by a plurality of radiation detection cells; a radiation detectiondevice for a nuclear medical diagnosis apparatus, comprising acoincidentality decision portion which selects two gamma rayscoincidentally detected by two of said radiation detection cells, and anadded-energy discrimination portion which discriminates the two gammarays in a case where an added value of energy values concerning said twogamma rays is equal to a predetermined value.
 9. In a radiationdetection method for a nuclear medical diagnosis apparatus havingradiation detection means for detecting gamma rays emitted from aradioisotope in a body of a subject, by a plurality of radiationdetection cells; a radiation detection method for a nuclear medicaldiagnosis apparatus, wherein: steps of said radiation detection meansinclude the step of discriminating and acquiring gamma rays whichimparted predetermined energy, and the step of discriminating andacquiring also gamma rays which caused a photoelectric effect afterhaving undergone Compton scattering once within said radiation detectionmeans.
 10. In a nuclear medical diagnosis apparatus having at least tworadiation detection means for detecting gamma rays emitted from aradioisotope in a body of a subject, by a plurality of radiationdetection cells; a radiation detection method for a nuclear medicaldiagnosis apparatus, wherein: in a case where the gamma rays are gammarays emitted in directions of 180 degrees at combination of a positronand an electron, and where coincidental counting acquisition is done forthe gamma rays coincidentally detected in two of said radiationdetection means, steps of said radiation detection means include thestep of discriminating gamma rays which imparted predetermined energy,so as to submit them to the coincidental counting acquisition, and thestep of discriminating also gamma rays which caused a photoelectriceffect after having undergone Compton scattering once within saidradiation detection means, so as to submit them to the coincidentalcounting acquisition.
 11. A radiation detection method for a nuclearmedical diagnosis apparatus as defined in claim 9 or 10, wherein saidradiation detection means is a semiconductor detector.
 12. A radiationdetection method for a nuclear medical diagnosis apparatus as defined inclaim 10, wherein, in a case where the Compton scattering and thephotoelectric effect are respectively caused in adjacent ones of saidradiation detection cells by the gamma ray, a step of said radiationdetection means includes the step of submitting said gamma ray to thecoincidental counting acquisition.
 13. A radiation detection method fora nuclear medical diagnosis apparatus as defined in claim 10, wherein,by the step of confirming energy quantities imparted to the radiationdetection cells through the Compton scattering and the photoelectriceffect, an incident position of the gamma ray on said radiationdetection means is specified.
 14. A radiation detection method for anuclear medical diagnosis apparatus as defined in claim 10, comprisingthe step of dividing said radiation detection cells into a plurality ofgroups, and the step of making the discrimination every group.
 15. Aradiation detection method for a nuclear medical diagnosis apparatus asdefined in claim 10, wherein, in the coincidental counting measurementand acquisition of the gamma ray which caused the photoelectric effectafter having undergone the Compton scattering once within said radiationdetector, the radiation detection cell as to which energy of said gammaray falls within a range of 166 keV through 300 keV is set as a subjectfor acquisition as an incident position of said gamma ray.
 16. In aradiation detection method for a nuclear medical diagnosis apparatushaving radiation detection means for detecting gamma rays emitted from aradioisotope in a body of a subject, by a plurality of radiationdetection cells; a radiation detection method for a nuclear medicaldiagnosis apparatus, comprising the step of selecting two gamma rayscoincidentally detected by two of said radiation detection cells, anddeciding coincidentality thereof, and the step of discriminating the twogamma rays in a case where an added value of energy values concerningsaid two gamma rays is equal to a predetermined value, anddiscriminating added energy of said two gamma rays.